沿平板下落薄膜流动的研究综述

沿平板下落薄膜流动的时空演化一直是流体力学中一个相当活跃的研究领域.全面回顾了下落薄膜从长波近似方程到积分边界层方程, 从线性稳定性分析到弱非线性分析, 从首次失稳到二次失稳以及从有限振幅计算到直接数值模拟的发展历程, 总结了下落薄膜已有的理论结果和数值结果.此外,还介绍了沿加热平板下落的薄膜流动的最新研究进展, 概述了其它类型下落薄膜的研究情况.      

Evaporation and flow dynamics of thin, shear-driven liquid films in microgap channels

Thin and ultra-thin shear-driven liquid films in a narrow channel are a promising candidate for the thermal management of advanced semiconductor devices in earth and space applications. Such flows experience complex, and as yet poorly understood, two-phase flow phenomena requiring significant advances in fundamental research before they could be broadly applied. This paper focuses on the results obtained in experiments with locally heated shear-driven liquid films in a flat mini-channel. A detailed map of the flow sub-regimes in a shear-driven liquid film flow of water and FC-72 have been obtained for a 2 mm channel operating at room temperature. While the water film can be smooth under certain liquid/gas flow rates, the surface of an intensively evaporating film of FC-72 is always distorted by a pattern of waves and structures. It was found, that when heated the shear-driven liquid films are less likely to rupture than gravity-driven liquid films. For shear-driven water films the critical heat flux was found of up to 10 times higher than that for a falling film, which makes shear-driven films (annular or stratified two-phase flows) more suitable for cooling applications than falling liquid films.     

Experimental investigation into three-dimensional wavy liquid films under the influence of electrostatic forces

Three-dimensional interfacial waves that develop on the free surface of falling liquid films are known to intensify heat and mass transfer. In this context, the present paper studies the effect of electrostatic forces applied to a falling film of dielectric liquid on its three-dimensional nonlinear wave dynamics. Therefore, measurements of the local film thickness using a confocal chromatic imaging method were taken, and the complex wave topology was characterized through photography. The experiments show a complex interaction between the electric field and the hydrodynamics of the falling film, whereby electrostatic forces were found to both increase and decrease wave peak height in different regions of the wave. Additionally, an electrically induced breakup of the three-dimensional wave fronts, which leads to a locally doubled frequency in streamwise direction, is found. The ability to influence the wave topology demonstrated here opens the possibility to optimize heat transfer processes in falling liquid films.     

Entropy generation in gas absorption into a falling liquid film

The study of mass transfer into falling films constitutes a significant aspect for numerous applications in the chemical technology and is considered the subject of many theoretical and experimental researches. Evaluating the second law of thermodynamics is one of the contemporarily used methods to determine the performances of an industrial process and to study various sources of irreversibility. Expressions of the liquid velocity, the gas concentration, the entropy generation rate as well as the main sources of irreversibility in the case of gas absorption (carbon dioxide) into a laminar falling viscous incompressible liquid film (water) without chemical reaction, are analytically derived and graphically presented and discussed.     

Entropy generation in gas absorption into a falling liquid film

The study of mass transfer into falling films constitutes a significant aspect for numerous applications in the chemical technology and is considered the subject of many theoretical and experimental researches. Evaluating the second law of thermodynamics is one of the contemporarily used methods to determine the performances of an industrial process and to study various sources of irreversibility. Expressions of the liquid velocity, the gas concentration, the entropy generation rate as well as the main sources of irreversibility in the case of gas absorption (carbon dioxide) into a laminar falling viscous incompressible liquid film (water) without chemical reaction, are analytically derived and graphically presented and discussed.     

Measurements of velocity,temperature and velocity fluctuation distributions in falling liquid films

The velocity, temperature and velocity fluctuation distributions within falling spindle oil films in an inclined rectangular channel were measured using hot-wire techniques and thin thermocouples. The interfacial shear was caused by cocurrent air flow.The results indicate that the liquid films are as a whole much more laminar-like than turbulent in a range of Reynolds numbers (4γ/μ) up to the experimental limit of 6000. Mixing motion occurs in the vicinity of the interface; however, the flow near the wall surface exhibits no sign of such eddy motions, as predicted by the wall law for single phase turbulent flow. Although velocity fluctuation is observed within films with interfacial shear, mean velocity profiles are approximately the same as those obtained by the laminar film prediction.     

On wave families in a two-layer falling film

Using an approximate method, families of non-linear steady-traveling periodic waves in a two-layer falling film have been found for the first time. Computed waves have qualitatively similar behavior as that of those found in homogeneous films but the quantitative characteristics of the waves strongly depend on additional similarity parameters in the two-layer films. In particular, the average location of the interface affects the bifurcation scheme of the waves.     

Experimental study of the falling film of liquid around a Taylor bubble

The present work reports an experimental study of the falling liquid film around single Taylor bubbles rising in vertical tubes filled with stagnant liquids by using a pulse-echo ultrasonic technique. The experiments were carried out in acrylic tubes 2.0  m long, with inner diameters of 0.019, 0.024 and 0.034  m, with five water-glycerin mixtures, corresponding to inverse viscosity number ranging from 15 to 22422. The rising bubble and the falling liquid film were measured by using ultrasonic transducers located at the one side of the tube. The velocity and profile of the Taylor bubble, and the development length and equilibrium thickness of the falling liquid film around the bubble were obtained by the ultrasonic signals processing. Based on the experimental results of the present study, several correlations available to estimate the equilibrium thicknesses of liquid films falling around Taylor bubbles were evaluated and new correlations were proposed to estimate the dimensionless equilibrium film thickness and the film development length respectively.     

Investigations of the Marangoni effect on the regular structures in heated wavy liquid films

Formation and development of quasi-regular metastable structures within laminar-wavy falling films were studied. These structures emerge within the residual layer between large waves and could be one reason for the break up of the falling film. The temperature field of the film surface was visualised using IR-thermography. The film thickness was obtained from point measurements with the chromatic confocal imaging method and converted into a film thickness field, based on a quasi-steady assumption and IR thermography images. The thermo-capillary nature (Marangoni effect) of the regular structures was proven experimentally.     

Wave frequency of falling liquid films and the effect on reflux condensation in vertical tubes

Visual investigations of effective wave frequency, structure and formation of isothermal and fully developed falling liquid films inside vertical tubes are presented without and with countercurrent flow of vapour. These are supplemented by novel transmissivity measurements leading to considerable improvement of the established wave shape and liquid film structure classifications. In addition, reflux condensation heat transfer data evaluated for the limiting case of zero shear stress are represented in terms of the wave factor. These are correlated with Reynolds and Kapitza numbers and they are interrelated with the observed wave characteristics.     

Luminescence technique for the measurement of local concentration distribution in thin liquid films

Experimental data of the concentration field with high spatiotemporal resolution is required for the comprehension of mass transfer increasing kinetic phenomena in falling liquid films. For this purpose a non-invasive measuring method based on luminescence indicators is developed. It is used to determine the concentration distribution and the local film thickness simultaneously. First results are presented for the oxygen absorption into a laminar-wavy water film flowing down a plane with an inclination angle of 4° and a liquid side Reynolds number of 177. With the measured concentration distributions the effective diffusion coefficients are calculated at three points in a single wave of the film.     

Wave formation on vertical falling liquid films

The method of integral relations is used to derive a nonlinear “two-wave” structure equation for long waves on the surface of vertical falling liquid films. This equation is valid in a wide range of Reynolds numbers and reduces to the known equations for high and low Re. Theoretical data for the fastest growing waves are compared with the experimental results on velocities, wave numbers and growth rates of the waves in the inception region. The validity of theoretical assumptions is also confirmed by the direct measurements of the instantaneous velocity profiles in a wave liquid film.     

Evolution of metastable quasi-regular structures in heated wavy liquid films

Formation and development of quasi-regular, metastable structures within laminar wavy falling films were studied using IR-thermography. These structures emerge within the residual layer between large waves. It is shown that the typical size of regular structures does not depend on the liquid flow rate and is in the order of magnitude of the critical length of the Rayleigh–Taylor instability. Also a model of thermal-capillary breakdown on the basis of a simplified force balance between the surface tension and the tangential stress as well as the energy balance in the residual layer is presented. Model predictions and experimental data are in good agreement.     

Pore-scale network model for drainage-dominated three-phase flow in porous media

Drainage displacements in three-phase flow under strongly wetting conditions are completely described by a simple generalisation of well understood two-phase drainage mechanisms. As in two-phase flow, the sequence of throat invasions in three-phase flow is determined by fluid connectivity and threshold capillary pressure for the invading interface. Flow through wetting and intermediate spreading films is important in determining fluid recoveries and the progress of the displacement in three-phase flow. Viscous pressure drops associated with flow through films give rise to multiple filling and emptying of pores. A three-phase, two-dimensional network model based on the pore-scale fluid distributions and displacement mechanisms reported by Øren et al. and which accounts for flow through both wetting and intermediate fluid films is shown to correctly predict all the important characteristics of three-phase flow observed in glass micromodel experiments.     

Numerical analysis on flow dynamics and heat transfer of falling liquid films with interfacial waves

Flow dynamics and heat transfer of falling liquid films with interfacial waves flowing on a vertical plate have been studied with originally proposed numerical simulation method. To discretize basic equations a staggered grid fixed on a physical space is employed. A small amplitude disturbance generated at inflow boundary develops to a solitary wave which consists of a large amplitude roll wave and small amplitude capillary waves. Instantaneous streamwise velocity profiles at the wave crest and trough are very different from a laminar flow. A circulation flow occurs in the roll wave and it affects temperature distributions, especially the strong effect is observed for high Prandtl number liquids. The interfacial wave enhances the heat transfer by two kinds of effects which are a film thinning effect and a convection effect. The dominating effect depends on the Prandtl number. Received on 23 December 1998     

Evaporation of pure liquids with increased viscosity in a falling film evaporator

The present study investigated fluid dynamics and heat transfer of viscous pure liquids in a falling film evaporator. This is of special benefit as it avoids mass transfer effects on the evaporation behaviour. Experiments at a single-tube glass falling film evaporator were conducted. It allowed a full-length optical film observation with a high-speed camera. Additionally the evaporator was equipped with a slotted weir distribution device. Test fluids provided viscosities ranging from μ = 0.3 to 41 mPa s. The Reynolds number was between 0.7 and 1,930. Surface evaporation and the transition to nucleate boiling were studied to gain information about the film stability at maximum wall superheat. A reliable database for laminar and laminar-wavy viscous single component films was created. The experimental results show a significant enhancement in the wave development due to the film distribution. A wavy flow with different wave velocities was superposed to the film in each liquid load configuration without causing a film breakdown or dry spots on the evaporator tube. It was found that nucleate boiling can be allowed without causing film instabilities over a significant range of wall superheat.     

Inhomogeneous deformations and pull-in instability in electroactive polymeric films

By means of a bifurcation analysis we show the onset of inhomogeneous equilibrium configurations in thin electroelastic polymeric films under assigned voltage. The resulting activation threshold decreases the diffusely adopted value obtained under the assumption of homogeneous deformations. We argue that the bifurcated inhomogeneous solution describes experimentally observed localization effects.     

Longitudinal flow characteristics of vertically falling liquid films without concurrent gas flow

Flow characteristics of liquid films vertically falling along the outer wall of a circular tube without concurrent gas flow are experimentally studied, and attention is given to the longitudinally developing liquid film flow in the flow direction. Flow measurements are carried out by the methods of needle contact and electric capacity, and the obtained data are statistically processed.There exists a definite difference in flow characteristics such as wave motion patterns, film thicknesses, critical Reynolds number, and so on, depending strongly on the longitudinal distance in the flow direction as well as the liquid film Reynolds number. Measured probability distributions of interfacial waves can be well expressed by the functions of probability distribution statistically well-known as normal, logarithmic normal and gamma distributions. In terms of these functions, interfacial wave patterns are definitely classified over the whole experimental flow regime. As a rule, interfacial wave motion proceeds vigorously with increases of the longitudinal distance and Reynolds number; however, there exists a flow condition that wave fluctuation never grows up but declines regardless of an increase of Reynolds number.